In European open patent application 00 51 136 a grinding machine is described with which several grinding processes can be performed simultaneously. For each different grinding process however a corresponding grinding tool with its own grinding unit is required. The greater the number of different grinding operations which must be performed on a workpiece, the greater must be the number of grinding tools which must be provided with this grinding machine.
In U.S. Pat. No. 4,587,763 of Hahn a tool machines the front face of the workpiece, that is the face directed toward the motor that drives the tool, and the inside face simultaneously. The tool does not project through the annular workpiece and machine its opposite face and, in fact, this would be impossible because the workpiece is resting on its back face on a turntable.
Another problem occurs with known grinding tools, even when they are specifically shaped to fit to a workpiece. When such a workpiece wears somewhat it no longer fits correctly and digs into the workpiece, ruining it.